The concept of using an optical system for the purpose of magnifying or resolving an image for the vision impaired has been with mankind since at least as early as the thirteenth century. Indeed, English scholar Roger Bacon, who is generally credited with the invention of spectacles recognized that myopia and hyperopi (nearsightedness and farsightedness, respectively) could be corrected by positioning lenses of varying strength between the eyes of the human observer and the image to be observed.
The strength of a lens is measured in diopters. The diopter of any given lens is obtained by calculating the reciprocal of the focal length of that lens (in meters). For example, if the focal length of a given lens is one meter, the lens has a strength of 1/1 or 1 diopter. Similarly, a lens having a focal length of 50 centimeters or 0.5 meter can be employed to provide a strength 1/0.5 or 2 diopters. As is readily apparent to skilled routineers in the art, the greater the diopter value of a given lens, the more powerful the lens.
As routineers in the art should also appreciate, the human eye includes, inter alia, an optical focusing system. This focusing system has a characteristic focal length defined by the cornea and the lens. The cornea and lens are adapted to cooperate to refract light reflected from a visually detectable image to form a pattern of information upon the eye's retina. The pattern of information formed upon the retina is then transmitted to the brain for recognition. The cornea-lens focusing system of the human eye has a focal length of about 0.016 centimeter which focal length results in a strength of about 60 diopters.
There are instances in which the retina of human being is located farther from the cornea-lens combination than the characteristic focal length thereof. In those instances, images of objects still form at the focal length of the cornea-lens combination; however, that focal length remains well in front of the retina, thus resulting in blurred (out of focus) vision. As the visually detectable object is moved close to the observer, the image is formed farther from (or behind) the focal length of the cornea-lens combination. Eventually the object can be moved sufficiently close to the observer that the image will fall upon the retina and result in the observation of a clear image. This situation describes an individual who is said to be nearsighted. Conversely, if a human being is farsighted, the retina is located at a point in front of the focal length of the cornea-lens combination.
Fortunately, both of these conditions (nearsightedness and farsightedness) can be readily corrected by employing eyeglasses equipped with lenses characterized by powers typically ranging from -15 to +15 diopters. It is to be noted that farsighted individuals require the use of a lens which decreases their inherent focal length; therefore a lens with positive diopters, i.e., a converging lens is employed. Nearsighted individuals require the use of a lens which increases their inherent focal length; therefore a lens with negative diopters is employed.
Unfortunately, advancing age occasionally makes it necessary to apply two different lenses for good vision correction; i.e., one lens for near field vision and one lens for far field vision. Rather than continually switching between two pairs of spectacles, American scholar Benjamin Franklin realized that two different lenses of different diopters, and therefore of different focal lengths could be employed in the same eyeglass frame. These eyeglasses were termed bifocals and generally include an upper lens portion which is adapted to correct for far field vision and a lower lens portion which is adapted to correct for near field vision. Today, such bifocals are routinely produced and in wide use.
Despite the usefulness and success of bifocal eyeglasses, certain objects, such as human readable information displayed by the dials and gauges on a automotive instrument panel may be disposed at a distance from the operator of the automotive vehicle which falls outside of and more particularly between the ranges of the effective correction provided by the upper and lower portions of bifocal lenses. While eyeglasses employing trifocal lenses (i.e., three lenses of different diopters in the same eyeglass frame) may be employed to correct this situation, the fabrication of trifocals lenses is quite costly. Further the us of trifocal lenses may be uncomfortable or disorienting to the user. This becomes especially true if the only time trifocal lens use is required is during the operation of an automotive vehicle.